Conventionally, there are methods such as the communication method called “MIMO (Multiple-Input Multiple-Output)” that increases data communication rate by modulating a plurality of sequences of transmission data and transmitting modulated data from a plurality of antennas at the same time. The receiving side receives the transmission signals from the plurality of antennas using a plurality of antennas.
Since the received signal determined at each receiving antenna is a plurality of modulated signals mixed in the propagation space, in order to reconstruct data associated with each modulated signal, a value of representing the fluctuation of each modulated signal on the propagation path (hereinafter referred to as “channel fluctuation”) needs to be estimated. For this reason, the transmitting apparatus inserts known signals such as pilot symbols in the modulated signals in advance and the receiving apparatus estimates the channel fluctuation between each transmitting antenna and receiving antenna on the propagation space based on the known signals inserted in the modulated signals. Each modulated signal is then demodulated using this channel fluctuation estimate value.
One of such methods is a method of carrying out inverse matrix calculation of a matrix made up of channel fluctuation estimate values and separating each modulated signal. There is another method whereby the positions of candidate signal points are identified using channel fluctuation estimate values, maximum likelihood detection (MLD) is carried out between the candidate signal points and received signal point and data transmitted by each modulated signal is thereby reconstructed (e.g., see Non-Patent Documents 1 to 3).
Such a communication technique using multiple antennas is disclosed, for example, in Non-Patent Document 1. Now, the contents disclosed in this Non-Patent Document 1 will be explained briefly using FIG. 1. Multi-antenna transmitting apparatus 30 inputs transmission signal A and transmission signal B to modulated signal generation section 3. Modulated signal generation section 3 applies digital modulation processing such as QPSK (Quadrature Phase Shift Keying) and 16QAM (Quadrature Amplitude Modulation) to transmission signals A and B, and sends out resulting baseband signals 4 and 5 to radio section 6. Radio section 6 applies radio processing such as up-conversion and amplification to baseband signals 4 and 5, and sends out resulting modulated signals 7 and 8 to antennas 9 and 10. In this way, multi-antenna transmitting apparatus 30 transmits modulated signal 7 of transmission signal A from antenna 9 and at the same time transmits modulated signal 8 of transmission signal B from antenna 10.
Multi-antenna receiving apparatus 40 inputs received signal 12 received by antenna 11 to radio section 13 and also inputs received signal 16 received by antenna 15 to radio section 17. Radio sections 13 and 17 apply radio processing such as down-conversion to received signals 12 and 16, and send out resulting baseband signals 14 and 18 to demodulation section 19.
Demodulation section 19 detects baseband signals 14 and 18 and thereby obtains received digital signal 20 of transmission signal A and received digital signal 21 of transmission signal B. Non-Patent Document 1 describes a method whereby demodulation section 19 carries out inverse matrix calculation of a channel estimation matrix and obtains received digital signals 20 and 21, and a method of acquiring received digital signals 20 and 21 by carrying out maximum likelihood detection (MLD).
Furthermore, Non-Patent Document 2 describes a method whereby, when reducing the amount of calculations by reducing candidate signal points in a demodulation section, the error rate performances is improved by carrying out iterative decoding. To be more specific, Non-Patent Document 2 describes a technique of carrying out re-coding using received signal points and canceled candidate signal points.    Non-Patent Document 1: “Multiple-antenna diversity techniques for transmission over fading channels” IEEE WCNC 1999, pp. 1038-1042, September 1999.    Non-Patent Document 2: “Studies on Application of Interleaving of Iterative Decoding using Signal Point Canceling in MIMO System—BER Characteristic in Rayleigh Fading Environment” IEICE, RCS2004-8, April 2008    Non-Patent Document 3: “Space Division Multiplex Scheme in MIMO Channel and Basic Characteristics Thereof” IEICE Transactions B, vol. J87-B, no. 9, pp. 1162-1173, September 2004    Non-Patent Document 4: “Likelihood detection utilizing ordering and decision partial bits in MIMO systems” IEICE Transactions on communications, vol. 89-B, no. 4, April 2006    Non-Patent Document 5: “Studies on Application of Likelihood Detection Method Utilizing Ordering and Partial Bit Detection in Space-multiplexing MIMO Systems to 64QAM” TECHNICAL REPORT OF IEICE RCS2006-30, May 2006 Non-Patent Document 6: “A comparison of optimal and sub-optimal MAP decoding algorithms in the log domain” IEEE ICC 1995, pp. 1009-1013, June 1995    Non-Patent Document 7: “Performance analysis and design LDPC-coded MIMO OFDM systems,” IEEE Transactions on signal processing, vol. 52, no. 2, February 2004    Non-Patent Document 8: “Likelihood function for QR-MLD suitable for soft-decision turbo decoding and its performance for OFCDM MIMO multiplexing in multipath fading” IEICE Transactions on communications, vol. E88-B, no. 1, January 2005    Non-Patent Document 9: “A universal lattice code decoder for fading channels,” IEEE Transactions on information theory, vol. 45, no. 5, pp. 1639-1642, July 1999    Non-Patent Document 10: B. Lu, G. Yue, and X. Wang, “Performance analysis and design optimization of LDPC-coded MIMO OFDM systems” IEEE Trans. Signal Processing., vol. 52, no. 2, pp. 348-361, February 2004    Non-Patent Document 11: B. M. Hochwald, and S. ten Brink, “Achieving near-capacity on a multiple-antenna channel” IEEE Trans. Commun. vol. 51, no. 3, pp. 389-399, March 2003    Non-Patent Document 12: S. Bäro, J. Hagenauer, and M. Witzke, “Iterative detection of MIMO transmission using alist-sequential (LISS) detector” Proc. of IEEE ICC 2003, May 2003    Non-Patent Document 13: B. M. Hochwald, and S. ten Brink, “Achieving near-capacity on a multiple-antenna channel” IEEE Trans. Commun., vol. 51, no. 3, pp. 389-399, March 2003    Non-Patent Document 14: S. Bäro, J. Hagenauer, and M. Witzke, “Iterative detection of MIMO transmission using alist-sequential (LISS) detector” Proc. of IEEE ICC 2003, May 2003    Non-Patent Document 15: P. Robertson, E. Villebrun, and P. Höher, “A comparison of optimal and sub-optimal MAP decoding algorithms in the log domain” Proc. IEEE ICC 1995, pp. 1009-1013, June 1995    Non-Patent Document 16: K. Kobayashi, Y. Murakami, M. Orihashi, and T. Matsuoka, “Varying interleave patterns with iterative decoding for improved performance in MIMO systems” Proc. of IEEE PIMRC2004, vol. 2, pp. 1429-1433, September 2004